Photopheresis, also known as extracorporeal photochemotherapy (ECP), is a medical treatment that involves the collection of a patient's blood, separation of white blood cells, and exposure of these cells to ultraviolet A (UVA) light in the presence of a photosensitizing agent. The treated cells are then returned to the patient's bloodstream. This procedure is used to treat various immune-mediated and malignant conditions, such as graft-versus-host disease, cutaneous T-cell lymphoma, and organ transplant rejection.